In the case of reforming a hydrocarbon base fuel, for example methane, into a hydrogen rich gas (endothermic reaction) through water vapor reforming, the operating temperature of the reformer is approximately 700° C. (outlet temperature of reforming catalyst layer); however, the surface temperature of the material composing the reformed is supposed to be equal or superior to about 1000° C. in the vicinity of the heating portion of the reformer. In addition, a hydrocarbon base fuel mixed with steam is introduced in the fuel reformer, and reformed into a gas containing hydrogen about 75%, carbon monoxide on the order of about 10 and several % and carbon dioxide on the order of about 10 and several %. Consequently, as for materials composing the fuel reformer, a material that can resist to such a high temperature and an environment containing steam, hydrogen, carbon monoxide, carbon dioxide, or others is required.
In the case of steel material containing Cr, generally, a steel material to be used under a high temperature is required to resist to the oxidation. In general, a dense oxide of Cr is formed on the surface, acts as protection film, and protects the base material. Moreover, the addition of Ni together with Cr increases the heat resistance and stabilizes also the structure. For instance, the stainless base material is an alloyed steel containing Cr and Ni for the purpose of improving the corrosion resistance, and is a steel containing Cr by about 11 mass % or more.
The stainless base material used for heat resistance has been improved by JIS on the basis of SUS302 of SUS 300s, and ordinarily, Cr concentration of stainless base material used for heat resistance is 15 mass % or more, and contained on the order of 25 mass % in SUS310S. The oxidation resistance is improved with the increase of Cr concentration; however, an abnormal setting occurs around 700 to 800° C. (950 to 1100 K), provoking degradation at the same time [a degradation phenomenon occurring together with the deposition of intermetallic compound σphase comprising mainly FeCr at 830° C. (1100K) or less is called sigma brittleness and tends to occur easily with the increase of Cr quantity].
On the other hand, in a high temperature atmosphere containing hydrocarbon base gas, carbon monoxide or carbon dioxide, C contained in the atmosphere penetrates into the steel (into the base metal), provoking cementation. As the cementation progresses, carbon binds with Cr in the steel, forms Cr carbide at the grain boundary, Cr concentration in the base material lowers, making difficult to form a dense Cr oxide on the surface and reducing the oxidation resistance and, at the same time, intergranular corrosive cracking might be provoked by the Cr carbide deposited on the grain boundary.
Therefore, in order to make the sigma brittleness resistance and the cementation resistance compatible, it is necessary to maintain the oxidation resistance all the way controlling the Cr concentration and, to limit the cementation. As mentioned above, inside the reformer, the compatibility of sigma brittleness resistance and cementation resistance is required.
The fuel reformer is used, for instance, to supply a fuel cell with hydrogen rich gas.
The fuel cell system of the related art has a scale of several tens KW or more, increasing the size of reformer for the reactor is used a thick material in order to make the sigma brittleness resistance and the cementation resistance compatible, provoking problems of heavy weight and high material cost.
Even a small reformer, conventionally, has been using a heat resistant alloy (SUS310S or the like) or super alloy (Incoloy800 or the like) of Cr:20 mass % or more and Ni:18 mass % or more; however, heat resistant alloy, especially super alloy such as Incoloy800 or the like of Cr:20 mass % or more and Ni:18 mass % or more is extremely expensive. Moreover, even in a reformer using a reaction tube material of super alloy, cementation phenomenon was observed in the vicinity of the catalyst layer after the reforming test (after 100 cycles, 800 hours), confirming its insufficiency.
On the other hand, a fuel reformer material containing C 0.1 wt % or less, Cr 17 to 23 wt %, Ni 8 to 29 wt %, Nb 0.1 to 0.4 wt %, containing one or two elements chosen from Ti, Zr by 0.05 to 0.3 wt %, and further containing N 0.02 to 0.05 wt %, B 0.003 to 0.01 wt % and composed of remaining Fe and inevitable impurities (Japan Patent Publication Laid-Open HEI 5-339679) is proposed.
Though this reformer material is excellent in creep rupture strength and oxidation resistance and generates quite scarcely a harmful σ phase, but does not consider the cementation, decreases Cr content in the base material as the cementation progresses as mentioned above, making difficult to form a dense Cr oxide on the surface and increasing the possibility of causing intergranular corrosion.
The object of the present invention is to provide a light, low cost and cheap, highly reliable and long life fuel reformer, that resolves conventional problems, and provides the sigma brittleness resistance and the cementation resistance, without increasing the wall thickness of the reformer in order to make the sigma brittleness resistance and the cementation resistance compatible.